Magnetic recording media, such as audio and video tapes, computer diskettes, etc., typically have a magnetizable layer coated on a substrate. The magnetizable layer frequently comprises magnetic particles in a binder. Particles of acicular .gamma.-Fe.sub.2 O.sub.3 (mag-hematite) and Fe.sub.3 O.sub.4 can be used in the magnetizable layer. These particles can be prepared by dehydrating goethite (.alpha.-FeOOH) to form hematite (.alpha.-Fe.sub.2 O.sub.3), reducing the particle to magnetite (Fe.sub.3 O.sub.4) and oxidizing at least a portion of the magnetite to .gamma.-Fe.sub.2 O.sub.3. The .gamma.-Fe.sub.2 O.sub.3 may be further modified with cobalt by known methods as desired. As demand grows for high recording density and high signal to noise ratios, the requirements for the magnetic particles become more stringent.
The magnetic particles should be of submicron size, be acicular with a large aspect ratio (major axis/minor axis), have a narrow particle size distribution, and be substantially free of dendrites (particle branching) or other irregularities. The size, shape, and distribution of the magnetic particles are directly affected by the size, shape, and distribution of the goethite precursor particles. Goethite particles which have a high aspect ratio (are acicular), are uniform in size, and are substantially free of dendrites will be more likely to provide magnetic particles with the desired properties.
Several processes are known for producing acicular goethite particles. These processes fall into two general categories, a high pH (or alkaline) method and a low pH method.
According to the alkaline method, a slurry of ferrous salt is reacted with an excess stoichiometric amount of an alkali hydroxide to form ferrous hydroxide. The ferrous hydroxide is then oxidized while the pH remains at levels greater than about 11. This high pH process produces goethite particles that have high aspect ratios. Controlling the oxidation profile may be used to affect size, shape and distribution of the particles. Unfortunately, these particles also tend to have a relatively large amount of dendrites and a particle size distribution which is too wide. Manufacturing costs also make this approach less desirable than the low pH method. A subset of the alkaline method is a "carbonate" type process as disclosed, for example, in JP 03-223120.
According to the low pH method, the goethite is prepared, at a pH less than 6, by reacting ferrous salt with less than a stoichiometric amount of an alkali compound (usually a hydroxide or a carbonate) to form a precipitated complex intermediate compound. When ferrous sulfates, chlorides, or carbonates are used this complex intermediate is called "green rust". See, e.g., A. Olowe and J. Genin, The Mechanism of Oxidation of Ferrous Hydroxide in Sulphated Aueous Media: Importance of the Initial Ratio of Reactants, Corrosion Science, vol. 32, p. 965-984 (1991). The intermediate compound is then oxidized to form goethite seed particles. Since less than a stoichiometric amount of alkali was added, iron(II) ions that did not react to form the complex intermediate compound are still available in the solution. These iron(II) ions are not affected by the oxidation reaction. However, with further addition of alkali compound, this iron(II) precipitates on the seed particles as an intermediate compound. The further addition of alkali compound and further oxidation, therefore, enables one to grow the goethite seeds to the desired size.
The size and shape of the final goethite particles depend upon the size and shape of the goethite seed particles. A popular method of controlling the size and shape of goethite seed particles has been the use of growth regulating agents, such as compounds of phosphate, silicate, zinc, nickel, and chromium, and carboxylicates. These growth regulating agents typically provide a more uniform size distribution, but unfortunately they also decrease the aspect ratio of the particles. The reduced aspect ratio leads to magnetic particles with lower coercive force (H.sub.c). Another method calls for the addition of acid compounds during seed formation which lowers the viscosity and narrows the particle size distribution. However, here too, there is a tradeoff between particle size distribution and aspect ratio (the narrower the distribution the lower the aspect ratio). Thus, there remains a need for a process which produces goethite particles with high aspect ratios as well as excellent shape and narrow size distribution.